This invention relates to the novel compound tin diiododeuteroporphyrin (SnI.sub.2 DP), to therapeutically useful compositions containing it, and to the use of the compound and the compositions in treating various metabolic afflictions of mammals, particularly humans.
Heme is a red pigment comprised of four subunits called pyrroles; these subunits are chemically joined to form a single large tetrapyrrole (porphyrin) ring structure. A metal atom is chelated at the center of this porphyrin. In higher organisms this metal is iron and the porphyrin-iron ring structure is called protoporphyrin IX or heme. In physiological systems heme is bound to certain proteins; these heme proteins bind oxygen at the site of the iron atom or they function as components of membrane bound electron transport systems. Cellular respiration, energy generation and chemical oxidations are dependent on these heme proteins.
In mammals and other vertebrates heme is oxidatively degraded by an enzyme called heme oxygenase to form the open chain tetrapyrrole biliverdin. Biliverdin is then reduced to bilirubin by another enzyme biliverdin reductase. In liver bilirubin is converted to mono- and di-glucuronide conjugates by the hepatic glucuronyl transferase system prior to its excretion.
Bilirubin is a toxic compound, but normally this toxicity is not manifest since bilirubin is rapidly bound to plasma proteins, transported to liver, conjugated and execreted. However in the newborn, undesirably high concentrations of bilirubin exist in serum and may produce neurotoxicity. The intractable neurological syndrome known as "kernicterus" is the most severe manifestation of bilirubin toxicity.
The basis of this neonatal hyperbilirubinemia lies in a number of factors, mainly the rapid hemolysis of fetal erythocytes after birth and a developmental immaturity of the hepatic conjugating system which normally facilitates the excretion of bilirubin via the bile. The levels of heme oxygenase, the rate of limiting enzyme in the catabolism of heme to bilirubin are also markedly elevated at this time resulting in high rates of production of this bile pigment. Current methodologies for suppressing severe neonatal jaundice include a. stimulation of the hepatic conjugating system for bilirubin by drugs, e.g. phenobarbital, b. partial or total exchange transfusion, and c. phototherapy. None of these methods is fully satisfactory since there are as yet many unanswered questions with respect to their safety. In addition all these methods are directed towards the disposition of bilirubin after it has been formed in the heme degradative sequence.
Elevated levels of bilirubin also often appear in the serum of individuals with diseases such as congenital anemias, thalassemia and sickle cell anemias as well as various forms of liver disease. The concentration of bilirubin in the serum of such individuals rarely reaches the high levels found in neonates. It does, however, attain levels which may be toxic and should be controlled.
It is therefore desirable to have available methods and materials to inhibit the catabolism of heme in order to prevent the accumulation of bilirubin in serum.
Copending and commonly assigned patent application Ser. No. 684,169, now abandoned, describes the use of tin protoporphyrin IX (SnPP) in the treatment of elevated levels of bilirubin in neonates and adults. Copending and commonly assigned patent application Ser. No. 715,515, now U.S. Pat. No. 4,657,902, describes the use of tin mesoporphyrin (SnMP) for the same purpose.
Maintenance of a proper equilibrium or balance of tissue heme content is essential to the normal physiological functioning of cells. When this equilibrium is disturbed by any condition characterized by excess heme degradation to bilirubin--as exemplified by the circumstances listed above--it would be clinically valuable to have a pharmacological mechanism for restoring the equilibrium state of heme in cells by facilitating the excretion of the excess amount of heme from the body.
In association with but independent of the conditions described above, excess iron also accumulates in the body and this accumulation of the metal over time can produce deleterious and even lethal consequences for the host. This excess of iron may derive from several sources; e.g. cooking methods (iron pots) or directly via the diet (e.g., iron-overload induced cutaneous porphyria), from excess therapeutic administration of the metal in an attempt vigorously to treat unresponsive anemias; from hypertransfusions to which certain patients with blood disorders are subject; idiopathically from the genetic and acquired disorders collectively known as "hemachromatosis"; from certain industrial exposures; but the most common causes of excess iron deposition in tissues, and the resultant pathologic consequences which derive thereof, are a consequence of common congenital hemolytic anemias such as sickle cell disease, the various forms of thalassemia, G-6-PD deficiency, hereditary spherocytosis and the like. In these disorders, a greatly shortened red cell life span results in continuous large depositions of iron in tissues to an extent exceeding the capacity of the body to re-utilize the metal. These tissue concentrations of iron rise to very high, toxic levels and lead to impairment of vital organ functions manifest, for example, by cardiomyopathy, pancreatic insufficiency (diabetes) and generalized endocrine failure.
There is no physiological mechanism for excreting this excess of iron and the only generally available therapeutic modality for this purpose is a pharmacological agent known as desferrioxamine. This agent is not specific for iron however and chelates other metals as well. It must, in order to be reasonably effective, be given intramuscularly and causes substantial local inflammation at the site of injection. Further, original suggestions that it was non-toxic have proved incorrect, and a large number of toxic reactions in treated patients have now been reported to occur after its use.
SnPP and SnMP as described in the copending and commonly assigned applications identified above both manifest the extremely advantageous properties of greatly enhancing the biliary excretion of iron into the intestinal contents where the metal is eliminated. SnPP and SnMP act in this additional fashion by blocking the binding of heme to heme oxygenase, thus preventing the release of iron which normally occurs in the process of heme catabolism and allowing one atom of iron to be excreted into the intestine with every molecule of uncatabolized heme.
Tryptophan is an essential amino acid which has profound effects on a number of metabolic pathways in the whole animal, including man, particularly in the nervous sytem. Tryptophan is metabolized principally in the liver. Tryptophan which is not metabolized in the liver accumulates in the plasma and in the brain. Brain levels of tryptophan are dependent on plasma levels of the amino acid which in turn are regulated by liver tryptophan pyrrolase. Tryptophan in the brain is metabolized by a different route than in the liver. One of the principal metabolic products of tryptophan in the brain is 5-hydroxytryptamine, or serotonin. The concentrations of tryptophan and serotonin in the brain are closely regulated in humans. Increased concentration of these products are associated with hepatic encephalopathy and migraine headaches. Encephalopathy is a known affliction characterized by degenerative changes in the brain cells leading to confused states and other abnormal behaviour patterns as well as convulsions, stupor and coma. Decreased concentrations of these products have been implicated in narcolepsy, depression and nyoclonic disorders characterized by uncontrolled jerky movements.
Tryptophan pyrrolase is a heme dependent enzyme which occurs in the liver of humans. It catalyzes the oxidative cleavage of tryptophan to N-formylkynurenine and is the first and rate-limiting enzyme in the catabolism of tryptophan in the liver. The active holoenzyme is normally about 50% saturated with heme, but fluctuations in the availability of cellular heme produce rapid changes in the enzyme activity by converting the inactive, heme-free apoenzyme to the active heme containing holoenzyme.
More specifically, an increase in the amount of heme in the liver such as can be produced by parenteral administration of SnPP or SnMP as a result of the ability of these compounds to block the catabolism of heme causes increased saturation of tryptophan pyrrolase as the active form of the enzyme. The increased activity of the enzyme resulting from its increased saturation with heme causes an increased rate of tryptophan metabolism in the liver. As a result there is less spill-over of intact tryptophan into the plasma and, ultimately, less accumulation of tryptophan and serotonin in the brain.
SnPP and SnMP, as will be apparent from the above, are very useful additions to the medical armamentarium. However, they both have the disadvantage that they are photosensitizing agents. When the therapeutic agent is administered it spreads throughout the body and, because, it absorbs light i.e. sunlight or light from ordinary fluorescent bulbs, causes skin rashes, flushed skin and general discomfort. It is, therefore, of significant medical interest to find agents which have the advantages of SnPP and SnMP without the disadvantage of their propensity to photosize when exposed to light in the long wave length ultraviolet region.